Traditionally, in the office equipment context, to make a copy of an original document, a light lens xerographic copier was used. In these machines, the image of the original is focused on an area of a photo receptor, which is subsequently developed with a toner. The developed image on the photo receptor is then transferred to a copy sheet to create a permanent copy of the original.
In recent years, there has been made available what is known as digital scanners. A digital scanner performs some of the same functions as a light lens copier such as illumination of the original document as well as focusing the image through lenses and mirrors (via an optical system). In a digital scanner, however, the original image is focused onto one or more photo sensitive sensor elements of an image sensor. The image sensor may be, for example, a charged coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device. The sensor elements convert the original image into a set of analog voltage signals that can then be further processed into digital signals. These digital signals can be temporarily retained in a memory and then eventually used to operate a printing apparatus when it is desired to print copies of the original. In some scanners, the digital signals may be used to create facsimiles or electronic mail (e-mail) for transmission to external computing devices or stored in a data base for later use.
One common type of digital scanner is the flat bed scanner. Flat bed scanners typically include a horizontal glass platen, an illumination system, an optics system and an image sensor. In one example of a flat bed scanner, the illumination system, optics system and image sensor are all located on a motor driven carriage below the platen. In order to scan a page, the page is placed, image side down, on the platen. During the scanning process, the illumination system projects a scanning light onto the page while the carriage is moved beneath the platen. Due to the movement of the carriage, the projected scanning light illuminates progressively each section of the page at a certain rate. For purposes of this application, the rate at which each section of a page is progressively illuminated by the scanning light is referred to herein as the “scan rate”.
As the projected scanning light illuminates each page section, the light reflected from the page section is focused (by the optics system) onto the image sensor. The image sensor in response captures the page section. This is accomplished as each individual sensor element of the image sensor reads the amount of light reflected from one point in the page section and transfers the information in the form of an analog voltage signal. The level of the voltage signal is proportionate to the number of photons of light that have struck the sensor element during the element's exposure.
In many flat bed scanners the actual scan rate can vary while a page is being scanned. This may be caused by changes in the ability of the scanner to process the data generated during the scanning operation or by human intervention. For example, during a scanning operation the scanner data buffer may reach capacity or the data transmission rate may be reduced. In either case, this can result in the scanner having to reduce the rate at which a page is being scanned.
Unfortunately, variability in scan rate can result in over exposure or under exposure of the image sensor during a scanning operation. If the image sensor is over or under exposed to a particular page section, the analog signals generated by the image sensor will not likely accurately represent the page section.
For example, if the scan rate drops below the standard scan rate while the image sensor is capturing a particular page section, this can result in the image sensor being over exposed to that page section. As a result, the sensor elements of the image sensor receive a greater number of photons while being exposed to this page section than what would be expected under proper exposure level conditions. This results in the image sensor generating a set of analog signals having voltage levels above the values that would accurately represent the page section.
On the other hand, if while capturing a page section, the scan rate goes above the standard scan rate this can result in the image sensor being “under exposed”. As a result, the sensor elements of the image sensor receive a lesser number of photons while being exposed to this page section than what would be expected under proper exposure level conditions. This results in the image sensor generating a set of analog signals having voltage levels values below the values that would accurately represent the page section.
Prior art flat bed scanners handle the affect of scan rate variability on the exposure level of the image sensor using a number of techniques. For example, some scanners will perform image processing routines to the signals (typically after the analog signals have been digitized) which were generated for page sections where the image sensor was over or under exposed. Unfortunately, these image processing routines can require valuable computational resources to execute and can increase the amount of processing time required by the scanner to process the data generated after a page has been scanned.
Other scanners will re-scan the page section where the image sensor was over exposed or under exposed. This second technique can require additional mechanisms which can add complexity and expense to the scanner. In addition, re-scanning a page section can significantly increase the amount the time required for the scanner to complete a scanning operation.
Accordingly, there has been a need for an improved way to handle the affect of scan rate variability on the exposure level of an image sensor in a digital scanner.